EVOLUTION BY

NATURAL SELECTION

• Darwin realized that a process much like artificial selection (domestication) happens in nature–Artificial Selection

Selecting for (intentional-controlled breeding) specific traits

• His Theory of Evolution by Natural Selection holds that evolution is the logical outcome of four postulates, which Darwin laid out in his introduction to Origin

• He considered the rest of the book one long argument in their support

• The 4 postulates, which apply to populations of organisms, are…

1. Individuals within populations are variable.

2. The variations among individuals, are, at least in part, passed from parents to offspring (inheritable).

3. In every generation, some individuals are more successful at surviving and reproducing than others.

4. The survival and reproduction of individuals are not random; instead they are tied to the variation among individuals. The individuals with the most favourable variations (adaptations), those who are better at surviving and reproducing, are naturally selected

• If these four postulates are true, then the composition of the population changes from one generation to the next

• The logic is clear: – if there are differences among the individuals in a

population that can be passed on to offspring, and if there is differential success among those individuals in surviving and/or reproducing, then some traits will be passed on more frequently than others

• As a result, the characteristics of the population will change slightly with each succeeding generation

• This is Darwinian Evolution: gradual change in populations over time

• Ex. Darwin’s Finches

• A few million years ago, one species of finch migrated to the rocky Galapagos from the mainland of Central or South America.

• From this one migrant species would come many -- at least 13 species of finch evolving from the single ancestor.

• The ecological niches exert the selection pressures that push the populations in various directions.

• On various islands, finch species have become adapted for different diets: seeds, insects, flowers, the blood of seabirds, and leaves.

• The ancestral finch was a ground-dwelling, seed-eating finch. • After the burst of speciation in the Galapagos, a total of 14

species would exist: – three species of ground-dwelling seed-eaters; – three species living on cactuses and eating seeds; – one living in trees and eating seeds; – 7 species of tree-dwelling insect-eaters.

• The finches differed mainly in the size and shape of their beaks.• Most recently, Peter and Rosemary Grant have spent many

years in the Galapagos, seeing changing climatic conditions from year to year dramatically altering the food supply.

• As a result, certain of the finches have lived or died depending on which species' beak structure was best adapted for the most abundant food -- just as Darwin would have predicted.